1. Field of the Invention
The present invention relates to an apparatus for and method of monitoring the number and power of optical signals contained in wavelength multiplexed signal light for use in a wavelength division multiplexing optical transmission system, as well as, to an optical transmission system employing such monitoring apparatus and method.
2. Description of the Related Art
To meet the social needs arising from the influx of the advanced information society, intensive research and development have been conducted on broadband high-speed communications such as image communications and long-haul communications such as international communications both utilizing an optical-fiber transmission network. A wavelength division multiplexing (WDM) transmission system for transmitting multi-channel optical signals having different wavelengths from each other through an optical-fiber line to realize high-speed broadband optical communications has been developed and put into practice.
For stable operation of a WDM transmission system, it is important that status of wavelength multiplexed signal light, such as number and power of optical signals contained in the wavelength multiplexed signal light, be held within a predetermined range. To that end, a wavelength multiplexed signal light monitoring apparatus is proposed for monitoring the signal light and obtaining monitored data of the signal light status at successive points in time.
Such a wavelength multiplexed signal light monitoring apparatus is described, for example, in Ref. 1 xe2x80x9cProceedings of the 1997 Communications Society Conference of the Institute of Electronics, Information and Communication Engineers, B-10-60, p. 359xe2x80x9d. This apparatus comprises a combination of an arrayed waveguide grating (AWG) and light receiving devices for detecting respective demultiplexed optical signals, thereby monitoring wavelength multiplexed signal light. Also, a module comprising an AWG and photodetectors all monolithically integrated on an indium phosphide substrate is described in Ref. 2 xe2x80x9cProceedings of the 1998 the Institute of Electronics, Information and Communication Engineers General Conference, C-3-113, p. 279xe2x80x9d.
The number of optical signals multiplexed in a signal light has been increased up to 100 so far. In the wavelength multiplexed signal light monitoring apparatus having the above-described structure, the plurality of optical signals (channels) contained in the wavelength multiplexed signal light are demultiplexed, and the light receiving devices are provided on a one-to-one relation to the respective channels for monitoring the presence or absence, and power of the optical signal for each of the channels.
Therefore, a large size AWG has been required and also the size of a corresponding light receiving device array has also been increased. In addition, the monitoring apparatus has been increased in size and has become complicated in its structure. Another problem is that the same number of light receiving devices as that of channels must have been provided, and hence high cost of the monitoring apparatus has become a problem.
It is an object of the present invention to provide an apparatus for and method of monitoring wavelength multiplexed signal light, that can obtain monitored data for multi-channel optical signals contained in the wavelength multiplexed signal light, and that may avoid a larger size and complicated structure and higher cost of the monitoring apparatus, thus enabling it to monitor easily the wavelength multiplexed signal light. The present invention also relates to an optical transmission system employing such monitoring apparatus and method.
In order to achieve this object, an apparatus is provided for monitoring wavelength multiplexed signal light containing multi-channel optical signals having different wavelengths from each other within a transmission band. The apparatus comprises an input optical waveguide; an output optical waveguide; an optical branching unit disposed between these two optical waveguides for branching a part of the signal light to a monitoring optical waveguide; a controllable optical filter installed on the monitoring optical waveguide to give a loss pattern to the optical signals; and a light detecting unit disposed at an output end of the monitoring optical waveguide.
The optical filter may be designed so that it can control the loss pattern depending on an electrical control signal. Such an optical filter may comprise a main waveguide for guiding the signal light from the optical branching unit to the light detecting unit; a sub-waveguide optically coupled to the main waveguide through a first optical coupler and a second optical coupler, the sub-waveguide constituting a Mach-Zehnder interferometer together with the main waveguide, the first and the second optical couplers; and a temperature adjusting unit for adjusting the temperature of at least one of the main and the sub-waveguides located between the first and the second optical couplers, thereby controlling the loss pattern.
The optical branching unit and the optical filter may be formed on the same substrate, as a planar lightwave circuit. Further, the optical branching unit may be an optical coupler having a coupling ratio of not more than 10%.
The apparatus for monitoring wavelength multiplexed signal light may further comprise an operating circuit for obtaining monitored data of multi-channel optical signals from the data of the wavelength multiplexed signal light detected by the light detecting unit for each of different loss patterns which have been set successively by the optical filter.
A method of monitoring wavelength multiplexed signal light containing multi-channel optical signals having different wavelengths from each other within a transmission band is also provided. The method comprises a step of branching a part of the signal light to a monitoring optical waveguide; a step of giving a loss pattern to the optical signals contained in the branched signal light; and a step of detecting the branched signal light to which the loss pattern has been given, wherein the loss pattern giving step and the light detecting step being performed several times while setting a plurality of loss patterns different from each other several times.
The method of monitoring wavelength multiplexed signal light may further comprise a step of obtaining monitored data of multi-channel optical signals contained in the wavelength multiplexed signal light, which have been detected in the detecting step.
By setting the number of loss patterns to be equal to that of optical signals, the monitored data of the respective optical signals may be obtained from the detected data for the respective loss patterns.
Furthermore, an optical transmission system is provided, the system comprising an optical transmission line through which is transmitted wavelength multiplexed signal light containing multi-channel optical signals having different wavelengths from each other within a transmission band; and a wavelength multiplexed signal light monitoring apparatus disposed on the optical transmission line for monitoring the signal light. The optical transmission system may further comprise a signal light control unit for controlling the status of signal light based on monitored data of multi-channel optical signals obtained by the apparatus.
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.